Carbetimer, an intermediate molecular-weight-derivatized copolymer of
maleic anhydride and
ethylene, has been shown to possess significant
antineoplastic activity in the stem cell assay. We have examined the antitumor activity of
carbetimer in vivo and in vitro against HM5-Carb/S and M21, both primary human
melanoma cell lines sensitive and resistant to
carbetimer, respectively. The mechanism of action of
carbetimer in HM5-Carb/S has been determined. Mice bearing palpable sensitive
tumors were treated with 10% lethal doses of
carbetimer (1500 mg/kg i.p.). The
tumor nucleotide profile was determined 4 hours later.
Uridine and
cytidine nucleoside triphosphates were reduced by 36.6 and 58.2%, respectively. In a similar experiment using
carbetimer-resistant
tumor, there was no change in the
tumor pool sizes of
uridine and
cytidine nucleoside triphosphate pools in
carbetimer- or saline-treated animals. Following 24-h exposure of the cells to 1000 microM concentration of
carbetimer, the
carbetimer-sensitive cells were pulsed with [14C]
uridine,
cytidine, or
thymidine for 30 min.
Pyrimidine nucleotides, in particular triphosphates, were reduced significantly as compared to the saline-treated control. Similar treatment of
carbetimer-resistant cells resulted in no change in the pool sizes of the
nucleotides. [14C]
Bicarbonate flux studies demonstrated that [14C]CO2 conversion into
UMP and
CMP was increased 200 and 140% of control in the
carbetimer-sensitive cells treated with 1000 microM
carbetimer; however, a similar treatment of the resistant cells showed no change in the pool sizes of the
nucleotide. Examination of
pyrimidine salvage
enzymes demonstrated that, in the sensitive cells,
carbetimer treatment reduced the specific activity of
uridine,
cytidine, and
thymidine kinase by 46, 37, and 60%. In a similar study using resistant cells, the specific activities were reduced 7 and 0%, respectively. In the restitution studies coincubation of
carbetimer-sensitive cells with
carbetimer and
uridine resulted in essentially the reversal of
carbetimer cytotoxicity. Thus,
carbetimer inhibits the growth of the sensitive cells by inhibiting the uptake and metabolism of performed
nucleosides both in vivo and in vitro.